This application requests support for the scientific program of the Second International Congress of Twin Studies. A meeting of broad biologic and medical interest is being planned, including symposium and contributed and invited papers on the epidemiology of twinning, methodologic approaches to the analysis of twin data, special problems related to twin pregnancies, and twin studies dealing with cancer, pharmacogenetics, behavioral genetics, and cardiovascular disease. Twin studies provide some of the most incisive approaches available for the analysis of complex human traits and are relevant to many different areas of investigation. The next two decades will witness increasing opportunities for the use of twins for biomedical research. The International Society for Twin Studies and a successful Second Congress will play an important role in the realization of these possibilities.